1. Field of the Invention
The invention relates to optical transceivers, and in particular to optical signal modulation techniques that provide a communications link between computers or communications units over optical fibers, such as used in high throughput fiber optic communications links in local and wide area networks and storage area networks.
2. Description of the Related Art
Communications networks have experienced dramatic growth in data transmission traffic in recent years due to worldwide Internet access, e-mail, and e-commerce. As Internet usage grows to include transmission of larger data files, including content such as full motion video on-demand (including HDTV), multi-channel high quality audio, online video conferencing, image transfer, and other broadband applications, the delivery of such data will place a greater demand on available bandwidth. The bulk of this traffic is already routed through the optical networking infrastructure used by local and long distance carriers, as well as Internet service providers. Since optical fiber offers substantially greater bandwidth capacity, is less error prone, and is easier to administer than conventional copper wire technologies, it is not surprising to see increased deployment of optical fiber in data centers, storage area networks, and enterprise computer networks for short range network unit to network unit interconnection.
Such increased deployment has created a demand for electrical and optical transceiver modules that enable data system units such as computers, storage units, routers, and similar devices to be optionally coupled by either an electrical cable or an optical fiber to provide a high speed, short reach (less than 50 meters) data link within the data center.
A variety of optical transceiver modules are known in the art to provide such interconnection that include an optical transmit portion that converts an electrical signal into a modulated light beam that is coupled to a first optical fiber, and a receive portion that receives a second optical signal from a second optical fiber and converts it into an electrical signal. The electrical signals are transferred in both directions over electrical connectors that interface with the network unit using a standard electrical data link protocol.
The optical transmitter section includes one or more semiconductor lasers and an optical assembly to focus or direct the light from the lasers into an optical fiber, which in turn, is connected to a receptable or connector on the transceiver to allow an external optical fiber to be connected thereto using a standard SC, FC or LC connector. The semiconductor lasers are typically packaged in a hermetically sealed can or similar housing in order to protect the laser from humidity or other harsh environmental conditions. The semiconductor laser chip is typically a distributed feedback (DFB) laser with dimensions of a few hundred microns to a couple of millimeters wide and 100-500 microns thick. The package in which they are mounted typically includes a heat sink or spreader, and has several electrical leads coming out of the package to provide power and signal inputs to the laser chips. The electrical leads are then soldered to the circuit board in the optical transceiver. The optical receive section includes an optical assembly to focus or direct the light from the optical fiber onto a photodetector, which, in turn, is connected to a transimpedance amplifier/limiter circuit on a circuit board. The photodetector or photodiode is typically packaged in a hermetically sealed package in order to protect it from harsh environmental conditions. The photodiodes are semiconductor chips that are typically a few hundred microns to a couple of millimeters wide and 100-500 microns thick. The package in which they are mounted is typically from three to six millimeters in diameter and two to five millimeters tall, and has several electrical leads coming out of the package. These electrical leads are then soldered to the circuit board containing the amplifier/limiter and other circuits for processing the electrical signal.
Optical transceiver modules are therefore packaged in a number of standard form factors which are “hot pluggable” into a rack mounted line card network unit or the chassis of the data system unit. Standard form factors set forth in Multi Source Agreements (MSAs) provide standardized dimensions and input/output interfaces that allow devices from different manufacturers to be used interchangeably. Some of the most popular MSAs include XENPAK (see www.xenpak.org), X2 (see www.X2msa.org), SFF (“small form factor”), SFP (“small form factor pluggable”), and XFP (“10 Gigabit Small Form Factor Pluggable”, see www.XFPMSA.org), 300 pin (see www.300pinmsa.org), and XPAK (see www.xpak.org).
Customers are interested in more miniaturized transceivers in order to increase the number of interconnections or port density associated with the network unit, such as, for example, in rack mounted line cards, switch boxes, cabling patch panels, wiring closets, and computer I/O interfaces.
Although these conventional pluggable designs have been used for low date rate applications, the objective of miniaturization often competes with increased data rate which is an ever-constant objective in the industry.
The increasing demand for higher data rates and greater throughput in optical fiber networks has created increased attention on a variety of techniques to modulate and encode digital data signals for transmission on optical fiber. One technique called wavelength division multiplexing (WDM) is the use of multiple wavelengths to carry multiple signal channels and thereby greatly increase the capacity of transmission of optical signals over the installed fiber optic networks. See, for example, Kartalopoulos, DWDM Networks, Devices, and Technology (IEEE Press, 2002).
In a WDM optical system, light from several lasers, each having a different central wavelength, is combined into a single beam that is introduced into an optical fiber. Each wavelength is associated with an independent data signal through the optical fiber. At the exit end of the optical fiber, a demultiplexer is used to separate the beam by wavelength into the independent signals. In this way, the data transmission capacity of the optical fiber is increased by a factor equal to the number of single wavelength signals combined into a single fiber.
In the optical transceiver, demultiplexing devices are typically designed to selectively direct several channels from a single multiple-channel input beam into separate output channels. Multiplexing devices are typically designed to provide a single multiple-channel output beam by combining a plurality of separate input beams of different wavelengths. A multiplexing/demultiplexing device operates in either the multiplexing or demultiplexing mode depending on its orientation in application, i.e., depending on the choice of direction of the light beam paths through the device.
In prior art WDM systems, data carrying capacity may be increased by adding optical channels. Conceptually, each wavelength channel in an optical fiber operates at its own data rate. In fact, optical channels can carry signals at different speeds. In current commercial systems, the use of WDM can push total capacity per fiber to terabits per second, although practical systems are closer to 100 Gbps. Generally, more space is required between wavelength channels when operating at 10 per second than at 2.5 per second, but the total capacities are nonetheless impressive. For example, in the case of four wavelength channels at a data rate per channel of 2.5 gigabits per second, a total rate of 10 gigabits per second is provided. Using eight wavelength channels at a data rate per channel of 2.5 gigabits per second, a total data rate of 20 gigabits per second is attained. In fact, other wavelength channels can be included, for example, 16, 32, 40 or more wavelength channels operating at 2.5 gigabits per second or 10 gigabits per second and allow much higher data throughput possibilities. Furthermore, it is also known in the prior art to use multiple optical fibers in a single cable or conduit to provide even higher transmission rates in a point to point link.
Although high throughput telecommunications networks do not constrain the size of the optical transceiver, optical transceivers for data center applications that use the Ethernet data communications protocol generally conform to IEEE 802.3 standard specifications and MSA form factors. Ethernet (the IEEE 802.3 standard) is the most popular data link network protocol. The Gigabit Ethernet Standard (IEEE 802.3) was released in 1998 and included both optical fiber and twisted pair cable implementations. The 10 GB/sec Ethernet standard (IEEE 802.3 ae) was released in 2002 with optical fiber cabling. Support for and twisted pair cabling was added in subsequent revisions.
The 10 Gigabit Ethernet Standard specifications set forth in the IEEE 802.3ae-2002 supplement to the IEEE 802.3 Ethernet Standard are currently the highest data rate that is standardized under the IEEE 802.3 framework. The supplement extends the IEEE 802.3 protocol and MAC specification therein to an operating speed of 10 Gb/s. Several Physical Coding Sublayers known as 10GBASE-X, 10GBASE-R and 10-GBASE-W are specified, as well as a 10 Gigabit Media Independent Interface (XGMII), a 10 Gigabit Attachment Unit Interface (XAUI), a 10 Gigabit Sixteen-Bit Interface (XSBI), and management (MDIO).
The physical layers specified include 10GBASE-S (R.W), a 850 nm wavelength serial transceiver which uses two multimode fibers. 10GBASE-LX4, a 1310 nm wavelength division multiplexing (WDM) transceiver which uses two multi-mode or single mode fibers; 10GBASE-L (R/W), a 1310 nm wavelength serial transceiver which uses two single mode fivers, and 10GBASE-E (R/W), a 1550 nm wavelength serial transceiver which uses two single mode fibers.
The 10-Gigabit media types use a variety of letters to represent the fiber optic wavelengths they use as well as the type of signal encoding used.
In the 10GBASE-X media types, an “S” stands for the 850 nanometer (nm) wavelength of fiber optic operation, an “L” stands for 1310 nm, and an “E” stands for 1550 nm. The letter “X” denotes 8B/10B signal encoding, while “R” denotes 66B encoding and “W” denotes the WIS interface that encapsulates Ethernet frames for transmission over a SONET STS-192c channel.
The 10GBASE-SR and 10GBASE-SW physical layer devices are designed for use over short wavelength (850 nm) multimode fiber (MMF). The design goal of these media types is from two meters to 300 meters of fiber distance, depending on the qualities of the fiber optic cable used. The 10GBASE-SR physical layer devices are designed for use over dark fiber, meaning a fiber optic cable that is not in use and that is not connected to any other equipment. The 10GBASE-SW media type is designed to connect to SONET equipment, which is typically used to provide long distance data communications.
The 10GBASE-LR and 10GBASE-LW physical layer devices are designed for use over long wavelength (1310 nm) single-mode fiber (SMF). The design goal of these physical layer devices is transmission from two meters to 10 kilometers (32,808 feet) of fiber distance, depending on cable type and quality (longer distances are possible). The 10GBASE-LR physical layer device is designed for use over dark fiber, while the 10GBASE-LW physical layer device is designed to connect to SONET equipment.
The 10GBASE-ER and 10GBASE-EW physical layer devices are designed with a 1550 nm optical signal for extended reach (40 km) over single-mode fiber (SMF). The design goal of these physical layer devices is transmission from two meters up to 40 kilometers (131,233 feet), depending on cable types and quality (longer distances are possible). The 10GBASE-ER media type is designed for use over dark fiber transmission, while the 10GBASE-EW media type is designed to connect to SONET equipment.
Finally, there is a 10GBASE-LX4 media type, which uses wave division multiplexing technology to send signals over four wavelengths of light carried over a single pair of fiber optic cables. The 10GBASE-LX4 system is designed to operate at 1310 nm over multi-mode or single-mode dark fiber. The design goal for this media system is from two meters up to 300 meters over multimode fiber or from two meters up to 10 kilometers over single-mode fiber, with longer distances possible depending on cable type and quality.
WDM high date rate applications have found widespread application in short reach Ethernet networks The difficulties associated with multi-gigabit signaling over existing wiring has limited the applications for such cabling, although efforts are currently underway for new copper cabling standards.
The use of course wavelength division multiplexing (CWDM) that utilizes just four optically multiplexed channels each transmitting a 3.125 Gb/sec signal over a single fiber pair (i.e. utilizing one fiber for each direction), is set forth in IEEE 802.3ae Clause 53 in the 10GBASE-LX4 Physical Media Dependent (PMD) sublayer. An optical transceiver designed for operating in conformance with such protocol is described in U.S. patent application Ser. No. 10/866,265, herein incorporated by reference.